MECHANICAL ENGINEERING DIVISION
SCHOOL OF ENGINEERING
COCHIN UNIVERSITY OF SCIENCE AND TECHNOLOGY

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ABSTRACT

The main function of condition monitoring is to provide the knowledge of machine condition and of its rate of change which is essential to the operation of this method. The knowledge may be obtained by selecting suitable parameters such as vibration for measuring and reading its value at intervals.
Signals from vibration sensors are measured and then compared with reference measurements in order that they may be interpreted. This involves some analysis of the signals ranging from simple RMS amplitude measurement to vibration signature or spectral analysis, possibly including waveform plots and extending in its most sophisticated form to data processing and a range of physico-mathematical concepts. Other techniques such as orbital analysis, time waveform and phase analysis have signifance as methods for study of particular dynamic characteristics.

Machinery distress very often manifests itself in vibration or a change in vibration pattern. Vibration analysis is therefore, a powerful diagnosis tool, and trouble shooting of major process machinery would be unthinkable without modern vibration analysis.
It is natural for machines to vibrate. Even machines in the best of operation condition will have some vibration because of minor defects as a result of manufacturing tolerances. Therefore each machine will have a level of vibration which may be regarded as normal or inherent.
When machinery vibration increases or becomes excessive, some mechanical trouble is usually the reason. Machinery vibration levels just do not increases or become excessive for no reason at all. Something causes it unbalance, looseness etc.
Each mechanical defect generates vibration in its own unique way. This makes it possible to positively identify a mechanical problem by simply measuring and studying its vibration characteristics.
The success of a process industry often depends on the continued, safe and productive operation of rotating machinery. An effective maintenance programme is vital to this kind of success. The quality of the company s maintenance programme determines how long the machines will run, how safe they are for the people working around them. The benefits of a good maintenance programme are:
1. Prolonged machinery life.
2. Minimizes unscheduled down time.
3. Eliminates unnecessary overhaul.
4. Eliminates standby equipment.
5. Provides more efficient operations.
6. Increases machinery safety.
7. Improves quality performance.
8. Improves customer satisfaction.

2. CONDITION MONITORING

The main function of condition monitoring is to provide the knowledge of machine condition and of its rate of change which is essential to the operation of this method. The knowledge may be obtained by selecting suitable parameters such as vibration for measuring and reading its value at intervals. With condition monitoring, repairs are carried out only when the condition of machine has deteriorated to a predetermined level. Thus repairs or replacement of parts take place only when it has definitely been proved that a fault exists and if it left unrepaired would result in unsatisfactory operation or breakdown with possible damage to other machine parts and disruption of production.

Rotary equipment will be classified into three categories depending upon critically for on-stream condition monitoring as described below.
A) Category-I(Critical Machines)
These are vital machines which will be generally of high cost, high speed, too large and complex in their design and duties and does not justify the economics of having another spare set and breakdown of which result in immediate and serious interruption in production.
These equipments will have continuous on-line vibration and bearing temperature monitoring systems. These machines will be monitored for vibration on bearing housings once a week using portable monitoring instruments.
B) Category-II (Semi-critical Machines)
These are essential machines which will be needed for normal operation of the plant, but having stand by set, and also the running speed of which will not be very high. Failure of such equipments will not cause immediate production loss as the stand by set will come in line in case of failure. These machines will be monitored for vibration on bearing housing once in two weeks. However, some of the important machines may be monitored once in a week depending upon the requirement and equipment behavior.
C) Category-II (Non-critical Machines)
These are desirable auxiliary and general purpose machines which, owing to its function, can be allowed to remain temporarily out of operation without having a serious effect on operations. These equipments will be normally having spare sets. These machines will be monitored for vibration housings once in a month.

4. VIBRATION

Vibration is simply the back and forth movement of an object from its position of rest. It is like an oscillatory motion. Vibrations in machines above certain limits are harmful to their functioning.
The most common causes of vibration are:
Unbalance of motor.
Looseness
Misalignment
Bend shaft
Eccentrical
Bad belt drive and drive chains
Electromagnetic forces
Hydraulic forces
4.1) Vibration Characteristics
Machines condition and mechanical problems are identified by simply noting its vibration characteristics are:
1) Amplitude (Displacement, Velocity, Acceleration)
2) Frequency
3) Phase

4.1.1) Vibration Displacement (peak to peak)
The total distance traveled by the vibrating part from one extreme limit of travel to the other extreme limit of travel is referred to peak to peak displacement. The vibration displacement is usually expressed in micrometer where one micrometer equals one thousandth of a millimeter (0.001mm).
4.1.2) Vibration Velocity
The velocity of the motion is definitely a characteristic of the vibration but since it is constantly changing throughout the cycle, the highest peak velocity is selected for measurement. Vibration velocity is expressed in millimeter per second peak.

Fig 4.1
Vibration velocity
4.1.3) Vibration Acceleration
Vibration acceleration is another important characteristic of vibration. Technically acceleration is the rate of change of velocity. It is normally expressed in g s peak, where one g is the acceleration produced by the force of gravity at the surface of the earth

Fig 4.2
Vibration Acceleration
4.1.4) Vibration Frequency
The amount of time required to complete one cycle of a vibration pattern is called the period of vibration. Vibration frequency is the measure of complete cycles that occur in a specified period of time.
Frequency = 1/period
The frequency of vibration is usually expressed as the number of cycles that occur in each minute or CPM (cycles per minute) or number of cycles per second or Hertz (Hz).
4.1.5) Vibration Phase
Phase is defined as the position of a vibrating part at a given instance with reference to a fixed point or another vibrating part. Phase measurements offer a convenient way to compare one vibration motion with another or to determine how one part is vibrating relative to another part.

Vibration Severity
There are no realistic figures for selecting a vibration limit which, if exceeded will result in immediate machinery failure. The events surrounding the development of a mechanical failure are too complex to set any reliable limits. On the other hand we must have some general indications of machinery condition that can be evaluated on the basis of vibration amplitude.

Fig. 4.3
General machinery vibration severity chart

On the fig 4.3 the horizontal axis is scaled in terms of vibration frequency and the vertical axis in terms of displacement. The area between the diagonal lines represents levels of vibration severity from extremely smooth to very rough.
The fig 4.4 is a severity chart which works much the same way but uses velocity and acceleration parameters and covers a higher CPM range

Fig.4.4
Vibration acceleration general severity chart

5. TRANSDUCERS

Transducer is a sending device which converts one form of energy into another. The Vibration Transducer (Pick-Up) converts mechanical vibration into an electrical signal. There are mainly three types of Vibration Transducers.
1) Velocity Transducers.
2) Accelerometer Transducers.
3) Proximity Transducers.
Velocity Transducer and Accelerometer Transducer are called Seismic Transducers. Proximity Transducer is called Non-contact Transducer.
5.1) Velocity Transducers
Velocity transducers respond directly to vibration velocity. Most vibration measurement instruments have provision for processing the electrical signal from a velocity pick up to show vibration displacement as well. In theory it is also possible to convert signals from velocity pickups to units of acceleration, however, this is not done in practice, because the results have been found to be unreliable.
5.1.1) Moving coil type

Fig 5.1
Moving coil type
The fig 5.1 is a simplified diagram of a seismic velocity vibration transducer. The system consists of a coil of fine wire supported by soft spring. A permanent magnet, firmly attached to the case of the transducer, provides a strong magnetic field around the coil. Whenever this transducer is fixed or held tightly against a vibrating object, this permanent magnet vibrates while the spring suspended coil of wire remains stationary in space. When the coil of wire cuts magnet lines of force, a voltage is generated in that wire. The voltage is proportional to the velocity of motion, the strength of the magnetic field, and number of turns of wire in the coil. The voltage generated is transmitted by cable to a vibration meter, monitor or analyzer.
5.1.2) Direct Prod Transducer
Many times it is necessary to measure the vibration of a small light weight part or structure. However, holding or attaching the standard velocity pickup to a small part can actually reduce the vibration. We can solve this problem by using a direct prod pickup such as the one shown in figure.

Fig 5.2
Direct prod transducer
The principle of operation of a direct prod pickup is identical to that of a seismic velocity pickup. With the direct prod pickup, a thin prod extends through the end cap of the pickup and is attached directly to the movable coil inside. To measure vibration with a direct prod pickup, we should fasten the main body of the unit to a rigid structure to serve as a point of reference. The tip of the prod is then attached to the vibrating part, using a threaded tip or a special magnetic tip. We should hold the direct prod unit by hand movements that naturally result; we should use an analyzer whose filter is tuned to the vibration frequency of interest. The low frequency vibration dye to the hand movements are thus eliminated from the measurements.
One of the advantages of the advantages of this type pickup is that it adds only the weight of the weight of the prod and moving coil to the vibrating part. This makes the pickup especially useful on small, light weight objects where the mass of a standard seismic velocity pickup can affect the actual vibration. It is often selected for use on balancing machines where parts may be balanced at speeds as low as 50 RPM with excellent results.

5.1.3) Piezoelectric velocity transducer
These transducers have an output that is proportional to velocity, but have no internal moving parts. Stresses due to vibrational forces applied to the pickup cause a crystal or special ceramic material to produce an electric charge. These are designed specifically for low frequency applications. It can measure down to 60 CPM.
5.2) Accelerometer Transducer
An accelerometer is a self generating devise with a voltage charge output proportional to vibration acceleration. Vibration acceleration is the measure of the rate of change of velocity and is normally expressed in terms of g s . Acceleration is a function displacement and frequency. As a result Accelerometer is extremely sensitive to vibration occurring at high frequencies.
5.2.1) Piezo-electric with built in amplifier

Fig 5.3
Accelerometer Transducer
The figure shows a simplified diagram of piezo-electric with built in amplifier. When this pickup is fixed or held against a piece of vibrating machinery, the mechanical vibrations are passed through the frame to a piezo-electric material. This material has the ability to generate an electrical charge in response to a mechanical force applied to it. In this instance mechanical vibration producers the force and the piezo-electric material responds by generating an electrical charge that is proportional to the amount of vibration acceleration.
5.3) Non-contact (Proximity) Transducers
Many high speed machines consist of relatively light weight motors mounted in massive cases and rigid bearings. Because of weight and stiffness of the massive machine case and bearings, externally mounted vibration and acceleration pickups often show little outward evidence of motor or shaft vibration. It is necessary to measure the actual shaft vibration in order to know when bearing clearances are in danger. It is displacement transducer measuring the shaft displacement relative to it fixing object.
5.4) Shaft Rider Accessory
A shaft stick is usable for periodic vibration checks and some analysis and in-place balancing operations. When it is necessary to monitor shaft vibration for extended
periods of time, it is recommended that we use a shaft rider.
The figure shows that a shaft rider is permanently installed in the bearing housing. It consists of a spring loaded probe that is held firmly against the rotating shaft so that is held firmly against the rotating shaft so that it accurately follows shaft motion.

Fig 5.4
Shaft rider accessory

6. VIBRATION ANALYSIS

Vibration Analysis is a two step process involving the ACQUISITION and INTERPRETATION of machinery vibration data. Its purpose is to determine the mechanical condition of a machine and specific mechanical or operational defects.
The Data Acquisition procedure is a means of systematic measuring and recording of the vibration characteristics needed to analyze a problem.
The Data Interpretation involves comparing the recorded data with the details of the machine, like its speed or speeds, its foundations, the construction details etc. then the characteristic of vibration typical of various defects are compared with the characteristics that have been measured. By this, one can pinpoint the trouble and take corrective measures.

7. DATA ACQUISITION

Data acquisition is the essential first step in vibration analysis, since the right data must be acquired under the right conditions to completely interpret a machine s condition.
Data acquisition can be done in several ways depending on the available instruments. Apart from data acquisition, additional data acquisition procedure such as semi-automatic, automatic and real time analysis are employed where the job can be quicker and more accurate.
In the semi-automatic method, the operator manually adjusts the filter through the frequency ranges, while the data is automatically recorded in a recorder. These types of plots are records of vibration amplitudes in the Y axis and the frequencies in the X axis. Such a plot is called Machinery Vibration Profile (Signature) and the analysis of the same is called as Signature Analysis.
Automatic data acquisition is the term used to describe the procedure of obtaining the data, where the instrument automatically plots the vibration profiles. This type of instrument incorporates and electronically swept filter as well as provisions for simultaneous plotting of data with the recorders.
7.1) Selection of Measurement Parameters
The various measurement parameters are displacement, velocity, acceleration:-
7.1.1) Displacement
Displacement can be measured with both velocity and acceleration pickups. This is accomplished by means of integrator circuits that are normally included in the circuit of vibration meters and analyzers. Pickups that respond directly to vibration displacement are readily available, but are usually used in the non-contact pickups.
7.1.2) Velocity
Velocity can also be measured with both velocity and acceleration pickups. Seismic and piezoelectric velocity pickups obtain vibration directly. The output from an accelerometer can be integrated to produce the equivalent of a velocity measurement, down to about 3Hz, or 180 CPM.

7.1.3) Acceleration
Acceleration should be measured only with an accelerometer. It is theoretically possible to differentiate signals from a velocity transducer to produce acceleration readings, but this would be needlessly complicated and expensive.
7.2) Common Types of Measurements
The common types of measurements are:-
1) Overall vibration amplitude measurements.
2) Amplitude Vs Frequency measurements.
3) Amplitude Vs Time measurements.
4) Phase measurements.
They are described below:-
7.2.1) Overall vibration amplitude measurements.
Overall vibration amplitude measurements provide a quick check of general machinery condition. A vibration meter or analyzer can be used for these measurements. This measurement is generally manually recorded in tabular form, or the data automatically stored in memory for computer based automated instruments.
7.2.2) Amplitude Vs Frequency measurements.
Amplitude Vs Frequency measurements provide frequency spectrum which is used to pinpoint the problem to a specific frequency or range of frequencies. Full capacity or advanced check analyzers are required to take these measurements. Data can be recorded manually in tabular form, or by semi automatic or automatic swept filter analysis with tabular or graphic hard copy recording of the data. FFT type analyzer can also provide tabular/graphic hard copy of visual display of the data.
It is estimated that over 85% of the mechanical problems occurring on rotating machinery can be identified by displaying the vibration Amplitude Vs Frequency data.
Importance of tri-axial readings
It is common practice to record the Amplitude Vs Frequency data measured in the horizontal, vertical and axial pickup directions at each bearing of the machines being analyzed. Obtaining measurements in all the three directions is extremely important for distinguishing between various mechanical problems. eg. Unbalance, Misalignment, bend shaft structural weakness (loose parts) will generally cause vibration at a frequency 1X RPM. Unbalance will almost always produce high amplitudes in the horizontal direction while lower amplitudes in the axial direction. Misalignment of couplings and bearings or a bend shaft will generally show relatively high amplitude of vibration in the axial direction. Amplitudes due to structural weakness, loose parts are shown in Vertical direction.

Fig 7.1
Vibration are normally taken in horizontal, vertical and axial directions on a machine bearing

Fig 7.2
Amplitude Vs Frequency
7.2.3) Amplitude Vs Time measurements.
Time measurements can be made during machine operation to detect vibrations that would not be apparent from Amplitude Vs Frequency analysis. Amplitude Vs Time measurements can be made for very fast transient vibrations or for slowly occurring vibrations. For fast transient vibrations use an oscilloscope with the horizontal axis scaled in milliseconds. For slowly varying vibrations use a recorder with the horizontal axis scaled in seconds. It can be taken with a DC recorder connected to an analyzer with that built-in-capability.

Fig 7.3
Short term Amplitude Vs Time data.

Fig 7.4
Long term Amplitude Vs Time data

7.2.4) Phase measurement
Phase measurements are important when analyzing mechanical problems in machinery. Phase is defined as the position of a vibrating part at a given instance with reference to a fixed point or another vibrating part. Phase measurements offer a convenient way to determine how one part is vibrating relative to another part.
To obtain phase measurements, an analyzer with a strobe light or remote reference pickup is required. The use of strobe light necessities visual observation of the rotating shaft and the capability to fire the strobe light with vibration signal in order to obtain phase. The remote phase pickup, which is usually an electromagnetic pickup, non-contact transducer or photocell must be installed so that to observe mechanical protrusion (depression) or a reflective mark on the shaft.
The strobe light measurement involves observing the angular position of the reference mark that appears under the strobe light, while the remote reference pickup provides phase readout (digital or analog) using a meter on the analyzer.

9. DATA INTERPRETATION

Once the necessary information have been collected by manual, or semi-automatic or automatic, the next step is to review and compare the reading with the characteristics of vibration typical of various types of troubles. A key to this comparison is the frequency. If a machine part has some defect, the frequency of vibration resulting from this defect will some multiple of the RPM. The multiple is different for different defects. Also there are some defects which will produce vibration frequencies that are not related with the RPM.
7.1) Causes Of Vibration
The major causes of vibration on Rotary machines are:-
1) Unbalance
The horizontal, vertical and axial vibration signatures presented in the figure given below illustrate typical Amplitude Vs Frequency analysis data resulting from an unbalance condition. It can be noted that, the predominant vibration occurs at 2200 CPM corresponding to the 2200 RPM fan speed. Since the amplitude of vibration in the axial direction is relatively low compared to the radial amplitudes, a bent shaft or misalignment is not indicated. The appearance of small amplitudes at the harmonic frequencies is common and does not necessarily indicate any unusual problems such as mechanical looseness.

Fig 7.1
Vibration amplitude Vs frequency data recorder identifies unbalance
2) Mechanical looseness
The vibration may be the result of loose mounting bolts, excessive bearing clearance, a crack or break in the structure or bearing pedestal, a rotor which is loose on the shaft, or some other loose machine component. The vibration characteristic of mechanical looseness will not occur unless there is some other exciting force such as unbalance or misalignment can result in large amplitudes of looseness vibration. The vibration due to looseness can be detected from Amplitude Vs Frequency when taking the reading in vertical direction.
3) Misalignment
Misalignment is an extremely common problem. Misalignment, even with flexible couplings, results in two forces, axial and radial vibration. The significant characteristic of misalignment and bent shafts is that vibration will be noted in both the radial and axial directions. As a result, a comparative axial vibration is the best indication of misalignment or a bent shaft.
4) Defective antifriction bearing
Flaws on the raceways, balls or rollers of rolling element bearings cause high-frequency vibrations and the frequency is not the multiple of the shaft RPM. The amplitude of vibration depends on the extent of the bearing fault. The natural frequency vibrations typically occur as vibration peaks in the 10,000 to 100,000CPM. Defects in the bearing components can generate vibration peaks at frequencies related to the bearing geometry. The vibration generated by the bearing is not normally transmitted to other points of the machine.

7.2) Recommended Method of Vibration Classification (As Per ISO 2372)
The machines are classified into five groups as per ISO 2372.
They are:
Class I
Individual parts of engines and machines integrally connected with the complete machine in its normal operating condition (Production electrical motors of up to 15KW are typical examples of machines in this category).
Class II
Medium sized machines (typically electrical motors of 15-75KW output) without special foundations rigidly mounted engines or machines (up to 300KW) on special foundations.
Class II
Large prime movers and other large machines with rotating mass mounted on rigid and heavy foundations which are relatively stiff in the direction of vibration measurement.
Class IV
Large prime movers and other large machines with rotating masses mounted on foundations which are relatively soft in the direction of vibration measurement (eg.Diesel-generator sets, especially those with light weight substructures).

Class V
Machines and mechanical drive systems with unbalancable inertia effects (due to reciprocating parts) mounted on foundations which are relatively stiff in the direction of vibration measurement.
Class VI
Machines and mechanical drive systems with unbalanceable inertia effects (due to reciprocating parts) mounted on foundations which are relatively stiff in the direction of vibration measurement. Machines with rotating slack-coupled masses such as beater shafts in grinding mills, machines like centrifugal machines with varying unbalances capable of operating as self contained units without connecting components, vibrating screens, dynamic fatigue-testing machines and vibration exciters used in process plants.

In practice, instead of good/Allowable/Just permissible, the following colloquial is used to stipulate the health condition of machines.
Good
Satisfactory
Just satisfactory
Unsatisfactory
Dangerous

Condition monitoring by vibration analysis gives information about the changing condition of the equipment thus enabling the avoidance of total breakdown and can have reduced time. Prediction of residual life enables the equipment to be stopped before they reach critical condition and thus safe operation is ensured. Improved production quality is achieved through condition monitoring. By condition monitoring, detection of incipient faults and determination of plant condition enable forecasting of maintenance demands.